Using Lion OS X.7.1, the Mouse file under System Preferences has done a runner. No longer appears and is not available under 'Show All' either. As a result wireless Mighty Mouse no longer scrolls and cannot reset preferences.

Any suggestion as the where this recalcitrant rodent might have moved house to?

Hang on to those original install discs like grim death! Using OS X.7 or later make a bootable USB thumb drive before running Installer!